1. Field of the Invention
The present invention relates to a reference voltage generator for generating a reference voltage within a semiconductor integrated circuit.
2. Description of the Related Art
A circuit used in a reference voltage generator in the related art is described with reference to FIG. 4. FIG. 4 is a circuit diagram of the reference voltage generator in the related art. A depletion NMOS transistor (hereinafter referred to as a D type NMOS transistor) 9 which is connected so as to function as a current source causes a constant current to flow into a diode-connected enhancement NMOS transistor (hereinafter referred to as an E type NMOS transistor) 10. By causing this constant current to flow into the E type NMOS transistor 10, a reference voltage corresponding to threshold voltages and sizes of the respective transistors is generated in the E type NMOS transistor 10.
First of all, a basic structure of the reference voltage generator is now described with reference to a schematic cross-sectional view of FIG. 5. The reference voltage generator includes a D type NMOS transistor) 9 and an E type NMOS transistor 10.
The D type NMOS transistor 9 which is connected so as to function as a current source includes a buried channel 12 so that the D type NMOS transistor 9 operates with a threshold value in a depletion region. In addition, a drain 17 is used as a power source terminal, and a gate electrode 13 and a source 16 are each connected to a reference voltage generation terminal. By adopting such a connection form, the D type NMOS transistor 9 described above functions as a constant current source. On the other hand, the E type NMOS transistor 10 which is diode-connected to the D type NMOS transistor 9 described above includes a surface channel 11 so that the E type NMOS transistor 10 operates with a threshold value in an enhancement region. In addition, a gate electrode 13 and a drain 15 are each connected to the reference voltage generation terminal, and a source 14 is connected to a ground terminal. That is, the D type NMOS transistor 9 and the E type NMOS transistor 10 are connected in series with each other. Therefore, when the D type NMOS transistor 9 and the E type NMOS transistor 10 are expressed in the form of an equivalent circuit, a circuit diagram illustrated in FIG. 4 is obtained.
Next, an operation of this reference voltage generator is described with reference to FIG. 3.
The D type NMOS transistor 9 described above operates as the constant current source. Therefore, for example, a drain current when a gate voltage with a grounded source is applied at regular intervals exhibits D type NMOS transistor characteristics 8 of FIG. 3 as transistor characteristics in this case. In FIG. 3, a threshold value of the D type NMOS transistor 9 is denoted by B, and the drain current is obtained at the gate voltage of 0 V. On the other hand, with respect to the E type NMOS transistor 10 described above, for example, a drain current when a gate voltage with the grounded source is applied at regular intervals similarly exhibits E type NMOS transistor characteristics 7 as the transistor characteristics. In FIG. 3, a threshold value of the E type NMOS transistor 10 is denoted by A. Here, the E type NMOS transistor 10 described above is diode-connected to the D type NMOS transistor 9 as the constant current source. Therefore, a gate voltage is required for causing a current to flow having the D type NMOS transistor characteristics 8 at the gate voltage of 0 V. This gate voltage is expressed by an output voltage C of FIG. 3 which becomes in turn an output from the reference voltage generator.
In the related art, as shown in Japanese Published Patent Application JP56-108258, the reference voltage generator is constructed in such a way that the D type NMOS transistor as the constant current source is operated in the depletion region by the buried channel, and the E type NMOS transistor diode-connected to the D type NMOS transistor is operated in the enhancement region by the surface channel. Here, the drain current characteristics for the gate voltage with the grounded source shown in FIG. 3 are especially important in the transistor characteristics. The drain current characteristics are electrical characteristics which are changed even by the change in temperature of the transistor. Because the temperature characteristics of the individual transistors constructing the reference voltage generator are different from one another, the temperature characteristics of the reference voltage generator are difficult to flatten over a wide temperature range.
In recent years, the improvement of the precision of an electronic apparatus has progressed, and the increased precision of an IC for controlling the electronic apparatus has been required. For example, in the IC, especially, a power management IC represented by a voltage detector or a voltage regulator, along with the miniaturization and the versatility of a portable apparatus to be loaded with the IC, it is required that even when a temperature is changed especially in the inside of the IC due to a change in ambient temperature environment, a reference voltage generator can generate a reference voltage with high precision, that is, temperature characteristics of the reference voltage become flatter.